As is well known in the art, the relative radial velocity of a target reflecting a radar pulse can be determined from differences in the phase shifts occurring in successive sweeps between the outgoing pulse and the incoming echo. The term "sweep" is meant to denote the time required by an outgoing pulse to reach the reflecting object and to return as an echo; within the so-called unambiguous range, this time does not exceed the recurrence period of the outgoing pulses.
In many instances the echoes to be suppressed are those generated by ground clutter which, in the case of a stationary radar, has a relative radial velocity and thus a Doppler frequency equal to zero. A variety of cancelers, referred to as moving-target indicators (MTI), are known in the art to eliminate such echoes, e.g. as described in the book titled "Introduction to Radar Systems" by Merrill I. Sholnik, Second Edition, published 1980 by McGraw-Hill Book Company and referred to hereinafter as Sholnik. In the simplest case, such a canceler includes a delay circuit introducing a lag equal to one recurrence period, the input and the output of that circuit being connected to respective inputs of a comparator generating an output signal only if the two input voltages differ from each other. It is also known to erase echoes from targets having relative speeds other than zero, as by the matching-weight technique described in commonly owned U.S. Pat. No. 4,217,584.
Since the determination of the relative phase shift between the outgoing pulse and the incoming echo requires the availability of information on the phase of the outgoing pulse upon the arrival of its echo, it is necessary to provide a continuous wave (CW) of stable frequency for this purpose. In a simple CW radar described by Skolnik, a stabilized oscillator energizes a transmitting antenna through a power amplifier which is controlled by a periodically operating pulse modulator. The same oscillator furnishes a reference signal to a receiver detecting the Doppler-frequency phase shift of the returning echoes. A typical power amplifier for a radar transmitter is a klystron whose structure, however, is considerably heavier, more complex and more expensive than that of a power oscillator such as a magnetron which is periodically triggerable to emit the outgoing pulses.
With such a power oscillator, as described on page 106 of Skolnik, it is necessary to utilize a coherent oscillator (COHO) which is locked in step with the oscillator at the beginning of each recurrence period. Owing to that periodic resynchronization, however, the COHO cannot be relied upon for phase-shift determination in the case of a sweep exceeding the recurrence period, i.e. with so-called second-time-around echoes. The phenomenon of such echoes generated by ground clutter is discussed in a book titled "Propagation of Short Radio Waves" by D. E. Kerr, published 1951 by McGraw-Hill Book Company.
Moreover, since the COHO must be changeable as to phase by a locking pulse during a brief interval on the order of, say, one microsecond, it cannot be tuned by a high-Q tank circuit so that its operating frequency may drift instead of remaining sufficiently stable to allow for a reliable distinction between clutter and moving targets located close to the far end of a large unambiguous range.
The erasure of second-time-around echoes from reflectors having unwanted relative velocities, such as stationary clutter, is also not feasible by conventional means when the pulse frequency or phase is deliberately varied from time to time in order to suppress certain blind speeds, as likewise discussed by Skolnik.